A) Field of the Invention
The present invention relates to a liquid crystal display apparatus.
B) Description of the Related Art
In a liquid crystal display apparatus in which (i) a TN liquid crystal cell in which liquid crystal molecules are horizontally aligned with respect to substrates and the alignment direction of the liquid crystal molecules is twisted by 90° between the upper and lower substrates, (ii) is disposed between two polarizers disposed in the manner of an approximately parallel Nicol arrangement so that an alignment direction of the liquid crystal molecules on a surface of one substrate may be in parallel to or orthogonal to an absorbing axis of the polarizers, a transmitted light intensity T is represented as the following relational expression (1).
                    T        =                              1            2                    ·                                                    sin                2                            (                                                π                  ⁢                                                            1                      +                                              u                        2                                                                                            2                            )                                      1              +                              u                2                                                                        (        1        )            
Herein, u is a value represented as following relational expression (2).
                    u        =                              2            ⁢            Δ            ⁢                                                  ⁢            nd                    λ                                    (        2        )            
In the relational expression (2), Δn indicates a refractive index anisotropy of a liquid crystal material (a birefringence), d indicates a distance between an upper substrate and a lower substrate (a thickness of a cell), and λ indicates a wavelength of the light to be incident on a liquid crystal display apparatus.
FIG. 5 is a graph showing the transmitted light intensities T of the relational expression (1) at λ=450 nm, 550 nm and 650 nm, respectively. The horizontal axis in the graph indicates Δnd in a unit “μm”, and the vertical axis indicates the transmitted light intensity T.
It can be seen to obtain a minimum value at which the transmitted light intensity T becomes zero (the first minimum obtained in the lowest Δnd, and after this, the second minimum and the third minimum are obtained as Δnd increases). Also it can be seen that the value of Δnd for giving the minimum value varies according to the wavelength λ of the incident light.
For this reason, in the normally-black TN liquid crystal display apparatus, it is considered that it is difficult to obtain an excellent darkness state of the achromatic color. Further, in the normally-white TN liquid crystal display apparatus of the related art, Δnd is generally set to the first minimum.
In the 90°-twisted TN liquid crystal cell, in a case where the retardation Δnd of the liquid crystal layer is very greater than the wavelength λ of the incident light (Δnd>>λ), linearly polarized light which is incident from a surface of one substrate of the liquid crystal cell is not changed in state of the polarized light over all of the wavelengths thereof within the liquid crystal layers and optically rotates by 90° to thus be emitted as linearly polarized light from a surface of the other substrate. This condition is referred to as the Mauguin condition. However, if Δnd does not increase significantly, it is difficult to obtain ideal characteristics, and thus this condition is not practical.
Herein, there have been proposed two types of configuration for the liquid crystal display elements of which typical examples are Japanese Unexamined Patent Application Publication No. 1-216318 and Japanese Unexamined Patent Application Publication No. 2001-147414, respectively.
Japanese Unexamined Patent Application Publication No. 1-216318 describes an invention for a normally-black TN liquid crystal display element in which much of fine unevenness is provided on the internal surface of at least one substrate, and the retardation Δnd of the liquid crystal layer is set to 1.4 μm to 2.2 μm, and the twisted angle is set to 60° to 150° respectively so as to improve the contrast and realize a background having a little coloration. Since the value of Δnd for giving the minimum value of a transmittance varies according to the wavelength λ of the incident light as described with reference to FIG. 5, cell areas having different thicknesses are distributed in the liquid crystal layers to realize the average background thereof and thereby, a processing for achromatic color of the background is performed.
Japanese Unexamined Patent Application Publication No. 2001-147414 discloses an invention for a TN liquid crystal display element in which the unevenness is not provided on the substrate surface, and the retardation And of a liquid crystal layer is set to greater than 2 μm. In this case, it is possible to realize a negative display in which a background transmittance is relatively low and the contrast is high.
As described in Japanese Unexamined Patent Application Publication No. 2001-147414, recently, a negative (normally-black) liquid crystal display element having a simpler configuration is widely used.
There has been proposed a method for improving viewing angle characteristics in the TN liquid crystal display element for the negative display (for example, Japanese Unexamined Patent Application Publication No. 2-125224). Japanese Unexamined Patent Application Publication No. 2-125224 describes a technique in which a pair of phase difference (retardation) plates are disposed between parallel Nicol polarizers disposed on the front and rear portions of, for example, a 90°-twisted TN liquid crystal cell interposed between the polarizers so as to arrange a slow axis of the phase difference plate in parallel to a transmittance axis of the polarizer adjacent to the phase difference plate, and thus it is possible to improve the viewing angle characteristics. In an embodiment of Japanese Unexamined Patent Application Publication No. 2-125224, a 90°-twisted TN liquid crystal cell is used in which the retardation And of the liquid crystal layer is 470 nm. However, a distribution of the refractive index in the used phase difference plates or a range for in-plane phase difference values is not described.